1. Field of the Invention
The present invention relates to a stepping motor used as a drive source for a lens drive and the like such as focus and zoom in a digital camera, a camera-fitted mobile telephone, and the like, and an electronic apparatus that includes the stepping motor.
Priority is claimed on Chinese Patent Application No. 200510108179.3, filed Oct. 9, 2005, the content of which is incorporated herein by reference.
2. Description of Related Art
Stepping motors have excellent compatibility with digital control systems, and are recently being frequently used as drive sources for a lens drive and the like such as focus and zoom in electronic apparatuses such as digital cameras and camera-fitted mobile telephones. Use of crow-pole type stepping motors is particularly frequent, since their manufacturing cost can easily be reduced and they are simple to control.
Various types of crow-pole stepping motor have been provided, such as, for example, a two-phase PM type stepping motor that includes two stators (A-phase stator and B-phase stator) incorporated in a cup-shaped housing (e.g., Japanese Unexamined Patent Application, First Publications Nos. 2004-289961 and 2001-78419) and a two-phase PM type stepping motor that does not use a housing (e.g., Japanese Unexamined Patent Application, First Publication No. H03-112356).
As shown in FIG. 7, for example, a two-phase PM type stepping motor 30 using a housing includes a rotor 32 consisting of a cylindrical permanent magnet 31 that is multipole magnetized in a circumferential direction R, an A-phase stator 33 and a B-phase stator 34 that are arranged so as to be mutually stacked along an axis L when disposed around the perimeter of the rotor 32, a housing 35 which the A-phase stator 33 and the B-phase stator 34 are accommodated inside of, and a bracket 36 that rotatably supports the rotor 32 after sealing the housing 35 with a lid.
The A-phase stator 33 and the B-phase stator 34 have identical configurations, including an exciting coil 42 formed by winding a coil winding 41 around the outer periphery of a ring-shaped bobbin 40 of resin material, and a pair of yokes 43 that enclose the exciting coil 42 from both sides along the direction of the axis L. A plurality of pole teeth 44 are formed in each of the pair of yokes 43, which enclose the bobbin 40 from both sides while being guided by a guide groove (not shown) formed in an inner peripheral face of the bobbin 40. At this time, the pole teeth 44 of the pair of yokes 43 are combined with the bobbin with their pitches deviate so that they sequentially arranged in a noncontacting state in the circumferential direction R. An inner peripheral face of the housing 35 contacts the outer periphery of the pair of yokes 43. Ends of the coil winding 41 wound around the exciting coil 42 are connected to external connectors 45 of the bobbin 40, enabling them to be electrically connected to an external component (not shown), such as a circuit component.
Incidentally, the external connectors 45 of the bobbin 40 are exposed to the outside via a notch (not shown) formed in the housing 35.
In contrast, as shown in FIG. 8, in a two-phase PM type stepping motor 50 that does not use a housing, one yoke unit 51 includes an inner yoke 52, a cup-shaped outer yoke 53, and an exciting coil 42; the yokes 52 and 53 are arranged so as to surround the periphery of the exciting coil 42. The stepping motor 50 is secured such that two of the yoke units 51 are enclosed from each side by a pair of brackets 54.
In FIG. 8, constituent parts which are identical to those of the stepping motor 30 shown in FIG. 7 are represented by identical reference symbols and are not repetitiously explained.
However, in the conventional stepping motors described in Japanese Unexamined Patent Application, First Publications Nos. 2004-289961 and 2001-78418 mentioned above, the following problems remain.
That is, in the stepping motor using a housing, because the housing 35 is constituted by the same members as the A-phase stator 33 and the B-phase stator 34 as shown in FIG. 7, the flow of magnetic flux from the rotor magnet of either one of the A-phase stator 33 and the B-phase stator 34 adversely affects the other.
This will be explained specifically using a pattern diagram of a magnetically equivalent circuit shown in FIG. 9. First, upon receiving magnetic flux from the rotor magnet, magnetic fluxes having a phase difference of 90 degrees flow through the stators 33 and 34. Ideally, the magnetic flux in one stator 33 (34) flows independently of that in the other stator 34 (33) and does not interfere with it.
However, in the conventional stepping motor 30, the magnetic circuits of the stators 33 and 34 become connected via the housing 35 as described above. In addition, contacting portions of their components inevitably have magnetic resistance. Magnetic resistance occurs at a total of five locations, namely locations (P1) where the pair of yokes 43 contact the housing 35 (four locations) and a location (P2) where the pair of yokes 43 surface-contact each other (one location).
Consequently, the flow of magnetic flux is not ideal, and increase in mutual magnetic interference between the stators 33 and 34 makes it difficult to reduce residual torque (detention torque). The resultant torque ripple makes it impossible to achieve smooth rotation, and problems such as vibration and noise are liable to arise.
In addition to the five locations of magnetic resistance, the direct flow of flux through the housing 35 makes the problem particularly noticeable.
On the other hand, with regard to the stepping motor 50 that does not use a housing shown in FIG. 8, although there is no direct flux flowing through the housing 35, as shown in FIG. 10, magnetic resistance occurs at a total of four locations, namely locations (P3) where the inner yoke 52 and the outer yoke 53 of the yoke units 51 contact each other (two locations), a location (P4) where both outer yokes 53 contact each other (one location), and a location (P5) where both inner yokes 52 surface-contact each other (one location).
Effects of mutual magnetic interference, while not as marked as in the stepping motor 30 using the housing 35, are nevertheless noticeable, and it is difficult to reduce residual torque. This is liable to result in similar problems such as vibrations and noise.